The present invention relates to circuits containing superconductive electrical switches used for interrupting persistent current flow in superconducting loops and also containing shunt resistors used to protect superconductive coils during quench events.
In superconductive electrical systems operating with a persistent current loop, current flows through superconducting coils and a superconducting switch. Interruption of current flow is accomplished by applying power to a superconducting switch in which a heater heats a portion of superconducting wire above its transition temperature, causing the superconducting switch to go normal. As the superconductive switch heats further, due to the I.sup.2 R power dissipation, where I is the current and R the resistance in the circuit, the resistance of the switch rises and therefore the voltage drop across the switch increases. To avoid having to dissipate all the stored magnetic energy of the superconducting coils in the switch, which would require a massive switch to limit the switch's temperature rise to an acceptable value to avoid superconductor wire damage, a string of shunt resistors is provided with a shunt resistor in parallel with each superconductive coil. As the resistance of the superconductive switch climbs with temperature, the voltage induced in each of the superconducting coils due to the L di/dt drop across the coil, where L is the inductance of the coil and di/dt the rate of change of the current through the coil, causes the power dissipation in the circuit to be shared by the superconductive switch and the shunt resistors. With proper sizing of the shunt resistors, the circuit can be designed to dissipate the bulk of the energy in the shunt resistors. With the resistance of each resistor in parallel with the coil made proportional to the coil inductance, the voltage drops across each resistor will exactly match the voltages across the coil sections when the same current flows through each resistor. Thus, with the same current flowing in each resistor section, the I.sup.2 R energy dissipation in each will be proportional to their resistance, and if their thermal masses are proportional to their resistances, the temperature rise of each resistor section will be identical with time.
However, if a quench of the superconductor initiates in one of the coils rather than in the switch, the various shunt resistors do not heat up uniformly since the majority of the magnetic energy is fed into the shunt resistor connected directly across the quenched coil. To minimize the weight of the shunt resistors needed to achieve a limited temperature rise, the resistors can be built as described in U.S. Pat. No. 4,568,908, entitled "Compact Resistor Assembly" issued Feb. 4, 1986. In U.S. Pat. No. 4,568,908 in one embodiment, the shunt resistor sections are electrically isolated but closely thermally coupled. Shunt resistors of this type can be difficult to manufacture due to the thin interplate electrical insulation used to maintain good thermal coupling between the different shunt resistors which can result in electrical shorts between resistors.
It is an object of the present invention to provide a circuit which reduces the heating of the parallel shunt resistor of a quenched coil when a quench occurs.
It is a further object of the present invention to provide a circuit which does not require direct thermal coupling between shunt resistors.